1. Field of the Invention
The present invention relates to devices for treating semiconductor wafers utilizing a plasma generated by electron cyclotron resonance.
2. Description of the Prior Art
In the production of semiconductor devices such as transistors and integrated circuits, semiconductor substrates or wafers are subjected to treatments, such as thin film formation, etching, oxidation, and doping. Recently, wafer treating devices utilizing a plasma generated by electron cyclotron resonance (ECR) have been developed, which have a number of advantages over conventional chemical vapor deposition devices, including low operating temperature and high treatment quality.
Japanese patent laid-open No. 56-155535 and U.S. patent application Ser. No. 315,730 teach a fundamental structure of such semiconductor wafer treating devices utilizing a plasma generated by electron cyclotron resonance. These ECR plasma wafer treating devices generally comprise a wafer treating chamber and a plasma generating chamber adjacent to the wafer treating chamber. Microwave energy having a frequency of 2.45 GHz generated by a microwave source is introduced into the plasma generating chamber through a waveguide. A solenoidal electromagnetic coil surrounding the plasma generating chamber generates a magnetic field of a flux density which corresponds to the microwave frequency so that electron cyclotron resonance is produced in the plasma generating chamber. Thus, absorbing the microwave energy, electrons in the plasma generating chamber are accelerated in a helical path. The collisions of these fast moving electrons generate the plasma of a gas in the plasma generating chamber, which is conveyed to the semiconductor wafer along the diverging lines of magnetic field produced by the solenoidal coil in the wafer treating chamber. In this manner, the treatment of the wafer, e.g. thin film formation or etching on the surface thereof, is effected. What kind of gas is utilized in the process, how great the gas pressure, and the level of the power of the microwave source, etc., are determined according to the type of treatment which is effected on the wafer.
The conventional ECR plasma wafer treating devices, however, have disadvantages as described in what follows. The above mentioned devices utilize microwave energy at the frequency of 2.45 GHz, the flux density of the magnetic field formed in the plasma generating chamber being regulated to 875 G to produce electron cyclotron resonance in cooperation with the microwaves. The Larmor radius of helically moving electrons, i.e. the radius of curvature of the projection of the helical paths of the electrons on a plane perpendicular to the direction of the magnetic field, depends on such factors as electron temperature and gas condition. However, the radius is proportional to the velocity of the electrons and inversely proportional to the cyclotron frequency: EQU Larmor radius of electrons=velocity of electrons/cyclotron frequency
Thus, assuming that the microwave frequency is 2.45 GHz and that the electron temperature is 100,000 K, the Larmor radius of electrons moving in helical paths in the plasma generating chamber in electron cyclotron resonance is about 0.1 mm. This Larmor radius of electrons is extremely small compared with the Larmor radius of electrons is extremely small compared with the radii of semiconductor wafers and of the plasma generating chamber, which are usually greater than 1 cm. Thus, the density of the plasma tends to be spatially uneven in the plasma generating chamber. As a result, the amount of the reactive ions reaching the surface of the semiconductor wafer in the wafer treating chamber also become uneven, thereby impairing the uniformity of the semiconductor wafer treatment.